General question on op-amp output resistors

[Woops is my middle name]

I have seen many pedals using op-amp output buffers that include a resistor to ground after the output coupling cap, often 100k, and sometimes also with a small value resistor in the audio output path, like the 470 ohm below.

I'm trying to understand the purpose of these - I guess the 100k ohm would make a high-pass filter with the 1uf cap, and the 470 ohm does.... what? I get having the cap there to isolate the op-amp DC from the audio path, but not sure why you'd need a high pass filter, as opposed to just doing a 1uf cap without the resistor.

If anyone has any insight, that'd be great! This is from the Sea Horse, if that has anything to do with anything.

 

[Robert]

R19 is an output pulldown resistor. It discharges any DC potential that might build up across C18.

 

[Woops is my middle name]

Ooook, I was wondering if it was a DC drain or something like that, cool. I guess the 470 ohm resistor is there just to prevent that leak from making it out to the cable?

Thanks!

 

[darwin999]

One role that R20 serves is that it safely limits the output current of IC1.3 in the event of a short circuited output

 

[Woops is my middle name]

Great, thanks for the explanations! It seemed like a best practice sort of thing so I figured it was for a reason...

 

[dcfvgb]

R19 also an output impedance to help signal transfer to whatever it plugs into
General rule of thumb: High input impedance, low output impedance.

 

[giovanni]

R19 also an output impedance to help signal transfer to whatever it plugs into
General rule of thumb: High input impedance, low output impedance.

The opamp typically has a low output impedance, which in parallel with a high resistor remains a low impedance so it shouldn’t affect the pedal’s output impedance substantially.

 

[JTEX]

R20 is mostly there to isolate the op amp output from the effect of a highly capacitive load, usually due to a long cable. Too much capacitance directly across the output can cause many op amps to become unstable (oscillate). R20 also allows passively summing the output with possibly another output coming from some other op-amp. Without series resistors on their outputs, you can't directly tie together two op-amp outputs - they would just fight and load one another.

 

[JTEX]

The opamp typically has a low output impedance, which in parallel with a high resistor remains a low impedance so it shouldn’t affect the pedal’s output impedance substantially.

Indeed. R19 has virtually nothing to do with the output impedance. Its only purpose is to gently drain any DC present on the right side of C20 to ground. Without it, DC would build up and cause a loud pop if the cap is suddenly discharged, for example through a bypass switch. In AC, the extra 100k load makes no difference to the opamp or the following stage.

 

[Woops is my middle name]

Brilliant. Thanks everyone for the input!

 

[mnemonic]

Does anyone know how important the value of R19 is in the above circuit? I’ve seen values from 3m3 down to 10k, does it affect output impedance in some way?


Reason I ask - I have built a simple TL071 buffer to fix an impedence mismatch, I am currently running an overly convoluted setup (for my own entertainment), tube amp into diy reactive load box, line-out tapped off output transformer into a mixer, FX blended using the mixer, then reamped with solidstate power amp into a cab.

I don’t know what the output impedence of my line out is (other than ‘high’), but my mixer has a 10k input impedance so a low output impedence buffer between line out and mixer input fixes the loading issue I initially had, and I would like to run it at high(ish) voltage to avoid clipping, so I don’t have to worry about how hot the signal is out of the line out.

At 9 volts supply a 100k pull down resistor works fine but at 30 volts supply I still get dc voltage on the output. Am I compromising anything about the circuit by using 50k or 10k pull down resistor?

 

[darwin999]

You need some DC on the output or you'll have no dynamic range for the signal - i.e., you don't want to completely short the output.
The dc voltage value of that segment is set by what it connects to on either side, including any capacitors that may be there.

I don't know your circuit, so it's difficult to say what sets the DC voltage in your case. But let me give a simple example of a capacitively coupled segment with no net current (so it's nominally static, hence it has zero electrical kinetic energy). If there's a voltage difference dV across a capacitor, it implies a charge imbalance dQ which has a potential energy cost of (1/2)*C*(dV)^2 = (1/2)*dQ*dV. The DC voltage of a capacitively coupled static segment will stabilize at the lowest total potential energy, which may be the sum of all the potential energies of the various capacitors.

 

[mnemonic]

You need some DC on the output or you'll have no dynamic range for the signal - i.e., you don't want to completely short the output.
The dc voltage value of that segment is set by what it connects to on either side, including any capacitors that may be there.

I don't know your circuit, so it's difficult to say what sets the DC voltage in your case. But let me give a simple example of a capacitively coupled segment with no net current (so it's nominally static, hence it has zero electrical kinetic energy). If there's a voltage difference dV across a capacitor, it implies a charge imbalance dQ which has a potential energy cost of (1/2)*C*(dV)^2 = (1/2)*dQ*dV. The DC voltage of a capacitively coupled static segment will stabilize at the lowest total potential energy, which may be the sum of all the potential energies of the various capacitors.

I’m afraid that second paragraph is over my head!

The circuit is the ‘stupidly awesome buffer’ by soulsonic on the freestompboxes forum.

 

[JTEX]

Does anyone know how important the value of R19 is in the above circuit? I’ve seen values from 3m3 down to 10k, does it affect output impedance in some way?

(also see post #9 above)

In the Stupidly Awesome Buffer, R5's value should be chosen so that the time constant of C3-R5 is not too long (probably not more than a second). Pick too high an R5 and it would take a long time before the DC output stabilizes down to where it should be (at zero!) after power is applied. 10uF and 100k yield a 1-second time constant. Personally, I'd go for less than 100k. Maybe 10k.

 

[mnemonic]

(also see post #9 above)

In the Stupidly Awesome Buffer, R5's value should be chosen so that the time constant of C3-R5 is not too long (probably not more than a second). Pick too high an R5 and it would take a long time before the DC output stabilizes down to where it should be (at zero!) after power is applied. 10uF and 100k yield a 1-second time constant. Personally, I'd go for less than 100k. Maybe 10k.

thanks, I think I understand a bit more now, how the value is decided upon.

 

[Big Monk]

I'm goofing around with different gain makeup configurations for my Electric Mistress clone board.

Here is the opamp gain stage:



I've also setup a transistor stage as well, and my curiosity based on this thread is around the difference in output impedance:



Now, it seems to me that the Opamp stage will have about a 100 ohm output impedance due to the 100 ohm resistor in series with the output.

A transistor stage actually has more parts but saves me some space on the board due to the size difference of a chip vs. transistor.

The only issue I see between the two is that the transistor stage is going to have an output impedance equal to 10k on it's own. I added the 100 ohm resistor in parallel to the R/C string on the end as that seemed to make sense to me.

Although I fear it may be a gross simplification.

My brain seems to think the following:

Z0 + R32 || R34 = Z0 New

10k + 100 || 100 = 99 ohm

Am I missing anything important or have I nailed it?